Carbon deposits on the tips of gasoline direct fuel injectors have been a problem since the introduction of the technology. The deposits result in flow reduction, spray variation, and increased particulate and hydrocarbon emissions. The problem has been partially resolved with careful configuration of the injector nozzles including hole inner diameter ratios, stepped holes, sac design, and coatings to reduce surface tension to discourage adhesion of the deposits, as disclosed, for example, in U.S. Pat. No. 6,502,769. The coatings have been for the most part ineffective. Flow and spray deviation has been mostly addressed with the mentioned configuration details.
The current trend toward centrally injected engines increases the problem, due to the higher injector tip temperatures encouraging deposit formation. There is still a lingering problem with injector tip surface deposits (not in the metering holes) that cause increased particle emissions. Particulate emissions are being strictly regulated beginning with Euro 6 and LEVIII legislation.
The current state of the art addresses flow deviation caused by deposits in the holes and not the deposits formed on the face of the injector tip. It has been determined that the deposits on the injector tip serve as a sort of sponge, storing fuel from the injection which later burns with a diffusion flame causing smoke and particulate emissions.
Thus, there is a need to keep the face of an injector tip free from deposits by ensuring that the fuel will oxidize on the tip surface before the deposits form.